The uses of inorganic particulate material are many and varied. Inorganic particulate material may, for example, be used as a filler or extender in numerous materials such as adhesives, sealants, glass, ceramics, rubber, paints, paper and plastics. The presence of an inorganic particulate material may provide advantageous properties such as colour, opacity, gloss, rheology, hardness, chemical resistance, thermal resistance or thermal conductivity. Calcium carbonate, for example, is often used in paints, plastics, film, adhesives and rubber.
Inorganic particulate materials are often stored and sold in aqueous suspensions. These aqueous suspensions may be stored for many days or weeks before they are used. The nature of the aqueous suspension may change over time, which may detrimentally affect the suitability of the aqueous suspension to be sold for an intended purpose.
For example, aqueous suspensions comprising an inorganic particulate material may be subject to pH change during storage.
For instance, the pH of an aqueous slurry of calcium carbonate generally decreases over time. Introduction of new calcium carbonate slurry product into a storage vessel over time may cause the pH of the calcium carbonate slurry to increase. Thus, the pH of the aqueous suspension may cycle during storage.
Without wishing to be bound by theory, it is believed that, in general, preparation of a calcium carbonate slurry forces carbon dioxide out of solution and produces an alkaline solution due to the presence of calcium and hydroxide ions. The calcium carbonate slurry may be stored in tanks for many days or weeks prior to delivery. During storage, the tanks may be stirred to prevent sedimentation and may also be recirculated (pumps circulate slurry from the bottom of the tank to the top). Drop in temperature of the slurry and exposure to air from stirring and recirculation contributes to atmospheric carbon dioxide dissolving back into the slurry. This causes the pH of the slurry to decrease (become more acidic) over time.
It is often important to maintain the pH of an aqueous suspension within a particular range so that it remains suitable for the purpose for which it is being sold. The pH of an aqueous suspension may also affect other properties of that aqueous suspension, which relate to the suitability of the aqueous suspension for its purpose. For example, the pH of a calcium carbonate slurry may affect its viscosity. At low pH values, the low shear slurry viscosity (Brookfield low shear) of the calcium carbonate slurry is higher.
It is thus desirable to prevent or reduce the rate of pH change of an aqueous suspension comprising an inorganic particulate material. For example, it is desirable to prevent pH decrease or reduce the rate of pH decrease of an aqueous suspension comprising inorganic particulate material over time. It may be beneficial to maintain the pH of an aqueous suspension within a particular pH range over a desired period of time.
Aqueous suspensions comprising inorganic particulate material may also be subject to contamination by microbes such as bacteria. This can negatively affect the properties of the aqueous suspension, for example by causing discolouration or malodour. It is thus desirable to prevent or limit the growth of microbes, or to reduce the number of microbes present in the aqueous suspension.